It is true that a common plain radiographic classification system uses cutoffs of <500 mL, 500 to 1,000 mL, and >1,000 mL.3 We agree that this is a logical starting point for a rule based on CT imaging. However, we avoided a method based on raw volume for two major reasons. First, we found a weaker association between absolute effusion volume and the CT imaging features, as well as inconsistencies in the relationship of absolute volume to percentage volume. For example, small effusions ranged from 78 to 328 mL, moderate effusions ranged from 378 to 1,566 mL, and large effusions ranged from 472 to 3,673 mL (data not previously shown). Second, we firmly believe that the physiologic effect of a pleural effusion of any given volume depends on the patient’s body habitus. It is hard to imagine that a 500-mL effusion in a small, elderly woman is as well tolerated as the same volume in a hulking male athlete. In fact, our data support this common sense conclusion, as the typical geometry of the layering effusion better reflects the volume percent rather than the raw volume. Thus, the effusion percent allows a better fit for the data and a more physiologic representation of disease severity.